Conventionally, electric actuators have been used as devices for controlling the opening/closing of valves used in air-conditioning equipment, plants, and the like. In these electric actuators, the opening/closing of valves has been controlled through the driving forces of motors, with ball valves or butterfly valves, or the like, installed as valve bodies in pipelines wherein cold water or hot water, high-temperature steam, or the like, flows.
One type of electric actuator is a spring-return-type electric actuator. In this spring-return-type electric actuator, a spring is provided, as emergency opening/closing means for the valve at the time of a power outage, on the valve shaft of the valve or on an output shaft that rotates the valve shaft, where the rotational force of the motor that rotationally drives the valve is used to wind the spring at the time of normal operation, so that when there is no rotational force of the motor at the time of a power outage, an opening operation or a closing operations to an emergency opening/closing position is performed by the biasing force of the spring that has been wound. Note that the emergency opening/closing position may be set as the fully closed position, or may be set as the fully open position.
However, even though such a spring-return-type electric actuator has the benefits of performing the opening operation or the closing operation with extreme rapidity at the time of a power outage of the valve, it also has the following problem areas: (1) being relatively noisy when opening/closing; (2) requiring a motor that produces a torque that is large enough to wind the spring at the time of normal operation, increasing the amount of power consumed; (3) the spring overrunning, that is, performing in the opening operation or closing operation beyond what is necessary, at the time of the emergency opening/closing, causing substantial damage to the equipment; and so forth.
Given this, there has been a proposal for an electric actuator wherein, instead of a spring, an electric double layer capacitor is provided in the electric actuator as a backup capacitor, where, during normal operation, the backup capacitor becomes electrically charged, and where, at the time of a power outage, the valve is forcibly opened or closed through driving the electric motor with the electrical energy that is stored in the backup capacitor. (See, for example, Japanese Unexamined Patent Application Publication 2007-218433.)
However, the electrical energy that can be stored in the backup capacitor depends on the electrostatic capacitance of the backup capacitor, where this electrostatic capacitance is reduced through variation over time, applied voltages, temperature fluctuations, and the like, and thus even if the terminal voltage of the backup capacitor is monitored so as to always maintain a voltage value that is set in advance, if, after some time has elapsed, a power outage were to occur, there would be the risk that the electrical energy that is stored may be inadequate, producing a state wherein it is not possible to perform the desired opening operation or closing operation (a return operation).
Moreover, because it is not possible to specify what the degree of opening of the valve will be at the time of the power outage, normally it is necessary to store in the backup capacitor the amount of electrical energy required to operate from a fully open state to a fully closed state or from a fully closed state to a fully open state. In contrast, the electric actuator does not always control the valve to be in the fully closed (degree of opening=0%) or the fully open (degree of opening=100%) position, but rather often the control is to an intermediate degree of opening. Notwithstanding only needing to operate to the fully closed or fully open state from the intermediate degree of opening when there is a power outage when the valve is controlled to an intermediate degree of opening, still the amount of energy required for moving from fully open to fully closed or from fully closed to fully open is stored in the backup capacitor. Because of this, an excessive amount of electrical energy is stored in the backup capacitor (it is overcharged), which not only interferes with energy conservation, but also reduces the service life of the backup capacitor.
Moreover, Japanese Unexamined Patent Application Publication 2008-89109 discloses charging a backup capacitor with only the electric power required for the opening operation or closing operation of the valve at the time of a power outage. However, based on the disclosure, even if the amount of electric power required for the opening operation or the closing operation to the emergency opening/closing position from the current opening position of the valve is calculated and the backup capacitor is charged with that amount of electric power, still there would be declining electrostatic capacitance of the backup capacitor due to changes over time, the applied voltages, variations in temperature, and the like, so that there would not always be an appropriate amount of electrical energy stored in the backup capacitor, with the risk that it might not be possible to perform the desired opening operation or closing operation (return operation).
Moreover, the electric double layer capacitor that is used as the backup capacitor has the distinctive feature of enabling rapid electric charging by a large electric current. However, when charged with a large electric current, heat is produced by the internal resistance within the electric double layer capacitor, which causes degradation of the electrostatic capacitance. Moreover, it is necessary to increase the output electric current-carrying capacity of the charging circuit, which increases costs.
The present invention was created in order to resolve such issues, and the object thereof is to provide an electric actuator that is able to perform reliably the desired return operation, regardless of when a power outage occurs, while always storing an appropriate amount of electrical energy in the backup capacitor. Moreover, it is to provide an electric actuator able to achieve a cost reduction through eliminating the need for increasing the output electric current-carrying capacity of the charging circuit along with preventing degradation of the electrostatic capacitance due to heating through the internal resistance of the backup capacitor.